Memory devices, whether they are volatile or non volatile, operate by storing, maintaining, and providing data. Of course, it is expected that memory devices should perform their duties reliably in that provided data should be the same data that was originally stored. However, over time, individual cells in memory devices can lose their ability to reliably store, maintain, and provide data. As a result, manufacturers often test their memory devices over a simulated lifetime in order to determine whether their memory devices provide an adequate life expectancy.
One non-volatile memory device in particular is a ferroelectric memory device, also referred to as ferroelectric random access memory (FeRAM). A ferroelectric memory device generally includes an array of memory cells where each memory cell contains at least one ferroelectric capacitor. The ferroelectric capacitor contains a ferroelectric material sandwiched between conductive plates. To store a data bit in a ferroelectric memory cell, a write operation applies write voltages to the plates of the ferroelectric capacitor in the ferroelectric memory cell to polarize the ferroelectric material in a direction associated with the data bit being written. A persistent polarization remains in the ferroelectric material after the write voltages are removed and thus provides non-volatile storage of the stored data bit.
A conventional read operation for a ferroelectric memory device determines the data bit stored in a ferroelectric memory device cell by connecting one plate of a ferroelectric capacitor to a bit line and raising the other plate to a read voltage. If the persistent polarization in the ferroelectric capacitor is in a direction corresponding to the read voltage, the read voltage causes a relatively small current through the ferroelectric capacitor, resulting in a small charge and voltage change on the bit line. If the persistent polarization initially opposes the read voltage, the read voltage flips the direction of the persistent polarization, discharging the plates and resulting in a relatively large charge and voltage increase on the bit line. A sense amplifier can determine the stored value from the resulting bit line charge or voltage.
Repeated reading and writing of a ferroelectric memory cell, which changes the polarization state of the ferroelectric capacitor, can fatigue the ferroelectric material and alter the properties of the ferroelectric memory cell. The resulting fatigue may eventually lead to a failure in that the ferroelectric memory cell is unable to maintain/store values. One mechanism employed to predict when a particular ferroelectric memory cell may fail is to measure the properties on the ferroelectric memory cell before and after performing a series of read and write operations on the ferroelectric memory cell. A measured change in the properties of the ferroelectric memory cell can then be extrapolated to the desired minimum life of the ferroelectric memory cell to project whether the ferroelectric memory cell will still be operable. If the extrapolation indicates that the ferroelectric memory cell will fail before reaching the desired minimum life, the ferroelectric memory cell may have a latent defect and may be replaced with redundant ferroelectric memory cells in a memory device.
The number of read or write cycles expected before a failure of a ferroelectric memory cell is relatively large (e.g., on the order of 10E15 cycles or more) in order to provide a memory device with a commercially viable life. The large number of cycles before failure can make fatigue testing time consuming. Thus, suitable life cycle testing could require about 10E12 to about 10E15 read/write cycles in order to simulate a lifetime of a ferroelectric memory cell. However, performing 10E12 read and write operations on every memory cell in a reasonable size ferroelectric memory device (e.g., in a 6-megabit ferroelectric memory device) would require days, weeks, or even months, making such testing impractical for production testing of ferroelectric memory device and at least bothersome when testing a ferroelectric memory device design. Extrapolation, which estimates a desired lifetime based on a shorter number of actual performed cycles, can be employed but can reduce the accuracy and/or reliability of the lifetime testing.